A problem concerned with pneumatically powered impulse wrenches is the difficulty to govern the tightening process accurately enough to ensure a correct and reliable pre-tensioning result. In a previously known impulse wrench system, described in U.S. Pat. No. 5,366,026, the output shaft of an impulse wrench is provided with a torque transducer for detecting the torque magnitudes of the delivered torque impulses, and a control unit for calculating a torque based clamping force and for initiating power shut-off as a certain co-efficient representing an increasing clamping force has reached a certain value. There is also described a way to more safely arrive at the desired final clamping force by reducing the motive pressure air supply to the impulse wrench as the difference between a desired final clamping force and the actual calculated clamping force is smaller than a predetermined value.
This known tightening system has two weak points from the reliability point of view, namely that the actual instantaneous tightening parameter values, like the torque magnitude, are obtained from an easily disturbed torque transducer including a magnetostrictive output shaft portion and electric coils mounted in the impulse wrench housing. This arrangement is not only sensitive to external disturbances resulting in a less reliable torque magnitude detection but it is rather space demanding and adds in a negative way to the outer dimensions of the impulse wrench. The magnetostrictive output shaft comprises a number of slots which weaken the shaft and call for an enlarged output shaft diameter.
Although this prior art patent describes a process control where the output torque of the impulse wrench is reduced as the clamping force magnitude approaches the target value, there is still a problem involved when tightening so called hard joints, i.e. joints having a steep torque growth characteristic. This is due to the fact that the very first impulse delivered by the impulse wrench could turn out to be powerful enough to cause a torque overshoot, i.e. reaching a torque magnitude that is higher than the desired final torque level. There is nothing described in this document about how to deal with this problem.
In WO 02/083366 there is described a technique for determining the installed torque based on signals delivered by an angle sensing means mounted on the inertia drive member of the impulse unit. This technique means that the delivered torque is calculated from the angular movements per time unit of the impulse unit, and that no torque sensing means on the output shaft is required. However, there is nothing described about how to control a screw joint tightening process by changing the output of the impulse wrench during the tightening process, for instance how to avoid over-tightening at the very first delivered torque impulse at hard joints.
In U.S. Pat. No. 6,668,212 there is described a method for tightening screw joints by means of a pneumatic torque delivering tool wherein the accuracy of the tightening results is improved by using calibration factors for power tool temperature, power tool age etc. and by varying the air inlet pressure to the power tool. This method is based on pre-production calibration procedures where the calibration factors for the actual screw joint and the different air pressure levels to be used during tightening are determined. Since this previous method does not use a power tool provided with torque sensing means the output torque of the tool has to be correlated to corresponding air pressure levels which are listed in a table, and when applying the power tool on a screw joint of a certain size the list tells the operator what air pressure levels should be used to safely achieve a desired final torque in the screw joint. Accordingly, this known method is not universally applicable on different screw joints but require a pre-production calibration procedure on the actual screw joint. This is disadvantageous in that it is complicated and time consuming.